1. Field of the Invention
The invention relates to vehicle driving force distribution devices that distribute a driving force received from an input shaft to first and second output shafts.
2. Description of the Related Art
Conventionally, a driving force distribution device is known which is placed on an auxiliary drive wheel-side in a driving force transmission system of a four-wheel-drive vehicle including main drive wheels to which a driving force of a driving source is constantly transmitted and auxiliary drive wheels to which the driving force of the driving source is transmitted according to the traveling state, and which distributes the driving force received from a propeller shaft to right and left wheels. See, e.g., Japanese Patent Application Publication No. 2013-154827 (JP 2013-154827 A).
In the driving force distribution device described in JP 2013-154827 A, a differential gear and a driving force interruption portion are accommodated in a differential carrier placed on a rear wheel-side of the four-wheel-drive vehicle. The differential gear includes in a differential case a differential gear mechanism formed by a pair of side gears and a pair of pinion gears. The driving force interruption portion includes a cylindrical clutch housing, an inner shaft accommodated in the clutch housing, a multi-plate clutch placed between an inner peripheral surface of the clutch housing and an outer peripheral surface of the inner shaft, and a pressing mechanism that presses the multi-plate clutch in an axial direction.
One of the pair of side gears is coupled to the right rear wheel, and the other side gear is coupled to the clutch housing of the driving force interruption portion. The left rear wheel is coupled to the inner shaft. The multi-plate clutch is formed by alternately arranging a plurality of outer clutch plates and a plurality of inner clutch plates in the axial direction. The plurality of outer clutch plates are engaged with the clutch housing so as not to be rotatable relative to the clutch housing. The plurality of inner clutch plates are engaged with the inner shaft so as not to be rotatable relative to the inner shaft. Lubricating oil is present between the outer and inner clutch plates in order to reduce abrasion and achieve smooth frictional sliding. When the multi-plate clutch is pressed by the pressing mechanism, the clutch housing is coupled to the inner shaft, and a driving force is transmitted to the right and left rear wheels.
In the four-wheel-drive vehicle described in JP 2013-154827 A, a dog clutch is placed on a front wheel-side of the propeller shaft. When the four-wheel-drive vehicle travels in a four-wheel drive mode, the dog clutch is coupled, so that a driving force of the driving source is transmitted to the propeller shaft and is transmitted from the propeller shaft to the right and left rear wheels via the differential gear and the multi-plate clutch of the driving force interruption portion which are located on the rear wheel side.
On the other hand, when the four-wheel-drive vehicle travels in a two-wheel drive mode, the dog clutch is decoupled, and the multi-plate clutch of the driving force interruption portion is disengaged, so that transmission of the driving force to the right and left rear wheels is stopped. In this case, transmission of the driving force from the driving source to the propeller shaft is cut off by the dog clutch, and transmission of a rotational force from the right and left rear wheels to the propeller shaft is also stopped. Rotation of the propeller shaft is therefore stopped even if the four-wheel-drive vehicle is traveling. Rotation of the differential case is also stopped accordingly. This reduces stirring resistance of the lubricating oil by gears of each part which is associated with the rotation of the propeller shaft and the differential case, and fuel economy of the four-wheel-drive vehicle can be improved.
According to the four-wheel-drive vehicle described in JP 2013-154827 A, the clutch housing and the inner shaft of the driving force interruption portion rotate in the opposite directions when the four-wheel-drive vehicle travels in the two-wheel drive mode. That is, one of the pair of side gears in the differential gear which is coupled to the right rear wheel rotates together with the right rear wheel, and the other side gear rotates in the opposite direction to the one side gear by rotation of the pinion gears. The inner shaft coupled to the rear left wheel rotates in the same direction as the one side gear coupled to the right rear wheel. The clutch housing and the inner shaft of the driving force interruption portion thus rotate in the opposite directions.
As the clutch housing and the inner shaft rotate in the opposite directions, drag torque is generated between the plurality of outer clutch plates and the plurality of inner clutch plates of the multi-plate clutch by viscosity of the lubricating oil. That is, when the four-wheel-drive vehicle travels in the two-wheel drive mode, this drag torque between the clutch plates serves as travel resistance, thereby causing power loss. The driving force distribution device described in JP 2013-154827 A has still room for improvement in this regard.